1. Field of the Invention
The present invention relates to an image forming apparatus including a charge bias application circuit for charging an image bearing member.
2. Description of the Related Art
Description is given below by taking a printer as an example of the image forming apparatus. Conventionally, the printer has a configuration as illustrated in FIG. 10A. A rotating polygon mirror 103 is rotated by a scanner motor 104. A laser beam 205 is emitted from a laser light source 207, and scans a photosensitive drum 201 serving as an image bearing member. A charge roller 202 uniformly charges the photosensitive drum 201. A developing roller (also referred to as “developing sleeve”) 203 develops an electrostatic latent image formed on the photosensitive drum 201 with toner. A transfer roller 204 transfers a toner image developed by the developing sleeve 203 onto fed paper. Fixing rollers 109 fuse and fix the toner image transferred onto the paper with heat. A cassette paper feeding roller 110 feeds the paper from a cassette to send out the paper to a conveyance path. Pairs of conveyance rollers 114 and 115 convey the paper fed from the cassette to a transfer position formed between the photosensitive drum 201 and the transfer roller 204.
FIG. 10B is a block diagram illustrating a circuit configuration of a control system for controlling the above-mentioned mechanical parts. Referring to FIG. 10B, a printer controller 501 loads image code data sent from an external device (not shown), such as a host computer, as bit data necessary for printing to be performed in the printer, and at the same time, reads and displays printer internal information. An engine control part 502 controls each part of the printer in response to an instruction from the printer controller 501, and at the same time, notifies the printer controller 501 of the printer internal information. A charge bias application circuit 206 controls, in response to an instruction from the engine control part 502, an output of a charge bias in a charge step among charge, development, and transfer steps. A laser driving circuit 505 controls ON/OFF of the laser light source 207 in response to an instruction from the engine control part 502.
FIG. 11 illustrates a schematic configuration of a charge bias application circuit part 601 for applying the charge bias to the charge roller 202 serving as a charge material for charging the photosensitive drum 201 serving as the image bearing member. The charge bias application circuit part 601 is an example of the above-mentioned charge bias application circuit 206. A voltage setting circuit part 602 is capable of changing a setting value according to a PWM signal. The PWM signal is input according to a target value of the charge bias to be output. A transformer drive circuit part 603 and a high voltage transformer part 604 are further provided. A feedback circuit part 605 detects a voltage value applied to the charge member/charge material (load) through a resistor R81, and transmits the voltage value to the voltage setting circuit part 602. In the subsequent control, a PWM signal (target value) is obtained so that the detected value is input, and a constant voltage is applied to the charge member/charge material (load). Through the control with such a configuration, a constant voltage can be applied to the charge member/charge material (load). For example, Japanese Patent Application Laid-Open No. H06-003932 discloses a high voltage power source device that employs such a technology of charge bias application.
However, a voltage for starting charging between the charge material (charge roller 202) and the charge member (photosensitive drum 201) changes depending on ambient temperature, a drum layer thickness, or the like. Hence, variations in voltage of the photosensitive drum 201 occur when the predetermined voltage is merely applied (FIG. 12A). FIG. 12A is a graph showing a relationship between an application voltage (V) applied to the photosensitive drum 201 and a drum voltage (V) of the photosensitive drum 201. In FIG. 12A, a circumstance H/H, a circumstance N/N, and a circumstance L/L represent that the state of the circumstance is high temperature and high humidity, normal temperature and normal humidity, and low temperature and low humidity, respectively. When an application voltage (Vout) is set constant, it is found from FIG. 12A that variations in voltage of the photosensitive drum 201 occur due to the difference in drum layer thickness or the difference in circumstance. From the fact that the sensitivity of the photosensitive drum 201 also differs due to the circumstance or the drum layer thickness, in a case where a laser beam with a constant light amount is emitted to the photosensitive drum 201, there also occur variations in voltage of the electrostatic latent image on the photosensitive drum after the laser illumination (FIG. 12B). FIG. 12B is a graph showing a relationship between a laser illumination light amount and a voltage (VL) of the photosensitive drum after the laser illumination. When the laser illumination light amount is set constant (for example, vertical chain line of FIG. 12B), it is found from FIG. 12B that variations in voltage (VL) of the photosensitive drum 201 after the laser illumination occur due to the drum layer thickness (in FIG. 12B, for example, −128 V in a case of thicker drum layer and −197 V in a case of thinner drum layer).
Further, as a characteristic of the photosensitive drum 201, drum memory adversely occurs through the laser illumination. The drum memory is a phenomenon that, though the drum voltage of the photosensitive drum 201 is supposed to be 0 V after a voltage remaining on the surface thereof is eliminated, the drum voltage becomes negative, resulting in variations in drum voltage after the laser illumination. In order to reduce the variations, the following measure has been taken. That is, a memory is provided to a process cartridge including the photosensitive drum 201, and, for example, a bias value according to the sensitivity and usage of the photosensitive drum 201 is stored in the memory. Then, based on the information, the charge bias, the developing bias, and the laser light amount corresponding to the sensitivity and the usage are corrected, to thereby reduce the variations in voltage. However, the control based on the information of the cartridge memory is predictive control. Therefore, as the printing speed or the cartridge toner amount is increased, the system using the predictive control based on the information of the cartridge memory has a limitation in the correction of the variations in voltages between Vd−Vdc and between Vdc−VL as shown in FIGS. 13A and 13B. In FIGS. 13A and 13B, Vd represents a drum voltage after the charging by the charge roller, Vdc represents a developing bias, and VL represents a drum voltage after the laser illumination.